The primary purpose of this case-control study is to determine whether exposure to 60-Hz magnetic fields in a residential setting and/or light- at-night (LAN) increases the risk of breast cancer in women who have regularly taken selected types of prescription medications (beta blockers, calcium channel blockers, psychotropics) in the ten years prior to diagnosis (or reference date for controls). The proposed study is prompted by specific findings in a study just completed by the research team (the melatonin study) that demonstrate effects of magnetic field exposure on nocturnal melatonin levels among women who are users of these classes of medication. A large case-control study of breast cancer in relation to residential magnetic fields and LAN, also just completed by the research team (parent study), did not evaluate the potential effects of such exposures on breast cancer risk among subgroups of women defined by the use of these medications. The present study is designed to obtain medication history data for women who participated in the parent case-control study so that more biologically directed analyses can be undertaken as suggested by the findings from the melatonin study. Women who participated in the parent study who are still alive and can be located will be invited to participate (anticipated to be approximately 1,471 women). Data collection will consist of a telephone interview designed to collect data on the use of the medications of interest (beta blockers, calcium channel blockers, and psychotropics). In an effort to disguise the primary purpose of the additional data collection, questions will also be asked about vitamin supplements and exercise habits, including the use of specific types of exercise equipment. Each subject will be sent an introductory letter describing the study and asking for her participation. Approximately three to five days after the letter has been mailed, a trained interviewer will call the subject and invite her to participate. If the subject agrees, a date and time for a telephone interview will be scheduled, and an advance packet of material will be scheduled, and an advance packet of material will be sent to the subject prior to the scheduled interview. The advanced packet will contain a letter describing the topic areas to be addressed in the interview, and materials to aid in the reconstruction and recall of medication use during the previous ten years. Conditional logistic regression analyses will be conducted to evaluate whether any relationship between the risk of breast cancer and exposure to magnetic fields or light-at-light varies among subjects with a history of using and not using any of the medications of interest.